γ-Fe2O3 Nanospheres with Good

ARTICLE pubs.acs.org/Macromolecules

Highly Conductive Polypyrrole/γ-Fe2O3 Nanospheres with Good Magnetic Properties Obtained through an Improved Chemical One-Step Method Zhiming Zhang,† Qiong Li,† Liangmin Yu,*,† Zhijie Cui,† Lijuan Zhang,*,‡ and Graham A. Bowmaker‡ †

College of Chemistry and Chemical Engineering, Key Laboratory of Marine Chemistry, Theory and Technology, Ministry of Education, Ocean University of China, Qingdao, 266003, P. R. China ‡ Polymer Electronics Research Centre, School of Chemical Sciences, University of Auckland, Private Bag 92019, Auckland, New Zealand ABSTRACT: An improved chemical one-step method (ICOSM) was used to prepare highly conductive and magnetic polypyrrole/γ-Fe2O3 (PPy/ γ-Fe2O3) nanospheres ∼80 nm in diameter. In the reaction process involved, FeCl3 acts as an oxidant for the polymerization of pyrrole and as a source of FeIII for the formation of γ-Fe2O3, which also requires the initial presence of FeII, provided by the addition of FeCl2. The method differs from the previous chemical one-step method (COSM) through the addition of p-toluenesulfonic acid (p-TSA) as the dopant after addition of the FeCl3 solution was finished. The conductive and magnetic properties of PPy/γ-Fe2O3 nanospheres increased and reached maximum values simultaneously with increasing amounts of FeCl2, contrary to the result observed in the previous COSM. The resulting electromagnetic PPy/γ-Fe2O3 nanospheres show maximum conductivity of 64.4 S/cm and saturation magnetization of 4.85 A m2/kg. The p-TSA dopant plays a critical role in the formation of PPy/γ-Fe2O3 nanospheres.

1. INTRODUCTION Electromagnetically functionalized micro/nanostructures of conducting polymers are of special interest due to their potential applications in areas such as electromagnetic interference shielding,1 microwave absorption,2 nonlinear optics,3 molecular electronics,4 and biomedicine.5 Previously, polyaniline (PANI) nanotubes containing Fe3O4 nanoparticles (∼10 nm in diameter) have been prepared by an in-situ chemical oxidation polymerization (two-step method) in the presence of Fe3O4 nanoparticles.6 The magnetic nanoparticles were prepared initially and were then incorporated in the oxidation polymerization of aniline to form the PANI/Fe3O4 nanotubes via a self-assembly process. We have also reported that coaxial PANI/γ-Fe2O3 nanofibers may be prepared by using γ-Fe2O3 nanoneedles as templates.7 The above in-situ methods suffer from the limitation that the added magnetic particles rapidly aggregate, and so it is difficult to disperse them uniformly in the PANI matrix. More recently, we developed a very simple chemical one-step method (COSM) to prepare PANI/γ-Fe2O3 nanofibers.8 The advantage of the COSM is that FeCl3 acts as the oxidant for polymerization of aniline and as a source of FeIII for the preparation of γ-Fe2O3 magnets, which results in a simultaneous preparation of the conducting polymer and the magnetic material, to form the electromagnetically functionalized composite nanofibers via a self-assembly process. By using this COSM, the magnetic γFe2O3 nanoparticles can be integrated uniformly in the conducting polymer matrix. Thus, it is a simple and economical route to r 2011 American Chemical Society

obtain electromagnetically functionalized composites in large quantity. However, all these electromagnetically functionalized PANI nanostructures show relatively low room-temperature conductivity (10
2
100 S/cm). In particular, good magnetic properties were usually accompanied by low electrical conductivity. Compared with PANI, polypyrrole (PPy) has a high conductivity, which encouraged researchers to synthesize electromagnetically functionalized PPy nanostructures using the above COSM. Thus, Xiao et al. have reported PPy/FeOOH nanospheres with a high conductivity of 16.1 S/cm by using the COSM.9 However, the maximum saturation magnetization only is 0.11 A m2/kg. Therefore, the preparation of a nanostructured conducting polymer composite with high electrical conductivity without compromising the magnetic properties is still a challenge. In the present study, an improved chemical one-step method (ICOSM) was proposed to prepare uniform electromagnetic PPy/γ-Fe2O3 nanospheres with high conductivity and good magnetization. In our previously reported COSM,8 no dopants were added into the polymerization solution. In order to get highly conductive composite nanostructures, p-TSA was added as dopant after the addition of FeCl3 solution was finished. Through this ICOSM, the conductivity and magnetization can be Received: March 21, 2011 Revised: May 17, 2011 Published: May 26, 2011 4610

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increased simultaneously by controlling the amounts of FeCl2, which is added as the source of FeII that is required for the formation of γ-Fe2O3.10 Of the resulting PPy/γ-Fe2O3 nanosphere products, the one that showed the maximum room temperature conductivity (64.4 S/cm) also had the highest saturation magnetization (Ms = 4.85 A m2/kg).

2. EXPERIMENTAL PART 2.1. Chemicals. Pyrrole monomer (Py) was distilled under reduced pressure. Other reagents, such as ferric chloride (FeCl3 3 6H2O), ferrous chloride (FeCl2 3 4H2O), p-toluenesulfonic acid (p-TSA), and ammonium hydroxide (NH4OH), were used as received without further purification. 2.2. Synthetic Method. A typical ICOSM procedure for the synthesis of electromagnetically functionalized PPy/γ-Fe2O3 composite nanospheres is as follows: Pyrrole monomer (0.133 mL, 2 mmol) was dissolved in aqueous ammonia solution (10 mL of 1.0 M) with ultrasonic stirring for 1 min to form white emulsion. Then a solution of FeCl2 3 4 H2O (30 mg, 0.014 mmol) in water (5 mL) was added to the above emulsion, to yield a pale blue solution. To this was added a solution of FeCl3 3 6H2O (3.24 g, 12 mmol) in water (10 mL), dropwise at a rate of 1 mL/min, whereupon the solution immediately turned brown. During the addition of the FeCl3 solution, the reaction solution was kept at 0
4 °C with stirring at the rate of 600 rpm. A quantity of p-TSA (0.34 g, 2 mmol) was added after addition of the FeCl3 solution was finished. The reaction solution was stirred for 12 h. The resulting product was collected by centrifuging at 4000 rpm for 5 min. The powder was washed with distilled water, ethanol, and ether three times, respectively. Finally, the powder was dried under vacuum at room temperature for 24 h. The molar ratio of p-TSA to Py ([TSA]/[Py]) and the amounts of FeCl2 3 4H2O added (expressed as [FeII]/[FeIII], the ratio of added [FeII] relative to initial [FeIII]) were varied to investigate the effect of the preparation conditions on the morphology and on the electrical and magnetic properties of the PPy/γ-Fe2O3 composites. 2.3. Measurements. The morphology of the products was characterized by scanning electron microscopy (SEM, Hitachi-530 or FESEM, JSM-6700F). The room temperature conductivity was measured by a Keithley 196 SYSTEM DM digital multimeter and an ADVANTEST R6142 programmable dc voltage/current source using a standard four-probe method. The chemical structure was characterized by FTIR, XPS, and X-ray diffraction. The FTIR spectra of the composites in KBr pellets were recorded using an IFS-113 V instrument. XPS data were obtained with an ESCALab220i-XL electron spectrometer using Al KR radiation under a base pressure of about 3  10
9 Torr. The binding energies were referenced to the C 1s line at 284.8 eV from adventitious carbon. X-ray diffraction was measured by a Micscience Model M18XHF diffractometer (MAC SCIENCE, Japan). Magnetization with an applied magnetic field (i.e., a hysteresis loop) at room temperature were carried out by using a vibrating sample magnetometer (VSM, Lakeshore 7307) with a maximum magnetic field of 795.77 kA/m.

3. RESULTS AND DISCUSSION 3.1. Electrical and Magnetic Properties of PPy/γ-Fe2O3 Nanospheres. In our previous COSM, the conductivity of

PANI/γ-Fe2O3 nanofibers is only as high as 1.2 S/cm. Although Xiao et al.9 reported that PPy/FeOOH nanospheres prepared by COSM can reach 16.1 S/cm, the saturation magnetization of 0.11 A m2/kg is too low. In the preparation process of both PANI/γ-Fe2O3 nanofibers and PPy/FeOOH nanospheres, increasing the FeCl2 amount results in more magnetic particles

Figure 1. Effect of the [FeII]/[FeIII] ratios on the conductivity and saturation magnetization of PPy/γ-Fe2O3 composite nanospheres.

formed, which is favorable for good magnetic properties. However, increasing the FeCl2 amount leads to decreased conductivity of the PANI/γ-Fe2O3 nanospheres. That is to say, it is hard to obtain the best electrical and magnetic properties at the same time using previous COSM. For our ICOSM, the enhancement caused by the FeCl2 is favorable for both the electrical and magnetic properties when the [FeII]/[FeIII] ratio is lower than 0.046, which can be seen clearly from Figure 1. The conductivity of PPy/γ-Fe2O3 composite nanospheres increases from 42.9 to 64.4 S/cm with an increase in the [FeII]/[FeIII] ratio from 0.014 to 0.046. At the same time, the saturation magnetization of PPy/ γ-Fe2O3 composite nanospheres also increases from 0.96 to 4.85 A m2/kg when the [FeII]/[FeIII] ratio increases from 0.014 to 0.046. Moreover, the conductivity and the saturation magnetization of PPy/γ-Fe2O3 nanospheres simultaneously reached maxima of 64.4 S/cm and 4.85 A m2/kg, respectively, when the [FeII]/[FeIII] ratio is 0.046. This positive increase of conductivity and magnetization with the increase of FeCl2 amounts in this ICOSM is contrary to that observed in the previous COSM, in which the conductivity of PPy/γ-Fe2O3 composite nanospheres decreased with an increase in the amount of FeCl2. All resulting PPy/γ-Fe2O3 composite nanosphere products in this ICOSM show high conductivity of 42.9
64.4 S/cm (as shown in Figure 1), which is greater by more than 3 orders magnitude compared with that of PANI/Fe3O4 nanotubes (∼10
2 S/cm)6 and by ∼50 times compared with those of the PANI composite nanofibers (1.2 S/cm) and PPy/Fe3O4 nanoparticles (1.0 S/cm) by using the previous COSM.8,11 Also, the conductivity of 42.9
64.4 S/cm for PPy/γ-Fe2O3 composite nanospheres obtained by using ICOSM is greater than that of PPy/γ-Fe2O3 composite (16.3 S/cm) prepared without p-TSA according to our previous COSM.8 All these significant results can be attributed to the addition of p-TSA after addition of the FeCl3 solution was finished. The reason that the conductivity of PPy/γ-Fe2O3 composite nanospheres increased with increasing the amount of FeCl2 may be that more Cl
will compete with TSA
to dope into the PPy main chain with increasing amounts of FeCl2 and lead to higher doping levels, which was confirmed by XPS data. As shown in Figure 2c, the binding energies of N 1s at 399.71 and 401.02 eV are observed, which are assigned to the undoped (
NH
) and doped (
Nþ
) state of PPy, respectively.12 The peak at 401.02 eV indicates that the PPy/γ-Fe2O3 composite nanospheres contain PPy in its doped state. The doping level can be calculated from the [
Nþ
]/[
NH
] ratio. The doping 4611

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Figure 2. X-ray photoelectron spectra for PPy/γ-Fe2O3 composite nanospheres prepared at the [FeII]/[FeIII] ratio of 0.046: (a) full survey spectrum; (b) expanded spectra of Fe 2p; (c) expanded spectra of N 1s.

Figure 3. Dependence of magnetization on the applied magnetic field at room temperature for PPy/γ-Fe2O3 composite nanospheres synthesized at different [FeII]/[FeIII] ratios: (a) 0.014, (b) 0.035, (c) 0.046, and (d) 0.058.

level of PPy/γ-Fe2O3 composite nanospheres increases from 28.07% to 37.24% when the [FeII]/[FeIII] ratio increases from 0.014 to 0.046, which is consistent with the increase of conductivity. In fact, the conductivity of PPy/γ-Fe2O3 composite nanopheres depends on two main factors: PPy and magnetic γ-Fe2O3 in the composite. Increasing the amount of FeCl2 enhances the conductivity of PPy because the doping level increased from 28.07% to 37.24% when the [FeII]/[FeIII] ratio increased from 0.014 to 0.046. At the same time, there is more magnetic γ-Fe2O3 produced with an increase in the amount of FeCl2, which will result in a decrease in the conductivity of the PPy/γ-Fe2O3 composite. There are thus two competitive processes that contribute to the conductivity of the PPy/γ-Fe2O3 composite with increasing FeCl2 amounts. In the earlier article,8 the conductivity of the polyaniline/γ-Fe2O3 nanofibers decreased with increasing amounts of FeCl2 because the decrease produced by γ-Fe2O3 exceeds the increase resulting from polyaniline. In our present ICOSM, it is obvious that the increase resulting from PPy exceeds the decrease produced by γ-Fe2O3.

Figure 4. SEM images of PPy/γ-Fe2O3 composites prepared at different [TSA]/[Py] molar ratios: (a) 0, (b) 1, (c) 2, and (d) 3.

The increase in the electrical and magnetic properties with increasing amount of FeCl2 is also different from the result reported by Zhang,13 in which the increased amount of FeCl2 resulted in higher magnetic properties but had no obvious effect on the conductivity of the PPy/Fe3O4 composite nanotubes. In Zhang’s one-step method, pyrrole monomer, p-TSA, and FeCl3 reacted for 2 h at first, and then FeCl2 and ammonia were added to the solution. This method was indeed a two-step method because it contained two completely separate reaction process. The polymerization of pyrrole was finished in the first 2 h, and magnetic Fe3O4 was formed when the FeCl2 and ammonia were added after the formation of PPy. So the addition of FeCl2 has little influence on the conductivity of PPy/Fe3O4 composite nanotubes. 4612

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Figure 5. SEM images and EDAX spectra of PPy/γ-Fe2O3 composites prepared at different [FeII]/[FeIII] ratios: (a, b) 0.014, (c, d) 0.023, (e, f) 0.046, and (g, h) 0.058.

Figure 3 shows that the PPy/γ-Fe2O3 composite nanospheres exhibit superparamagnetic behavior (i.e., no hysteretic loop), and the saturation magnetization of the PPy/γ-Fe2O3 composite nanospheres increases from 0.96 to 4.85 A m2/kg with an increase in the [FeII]/[FeIII] ratio from 0.014 to 0.046. This is because there are more γ-Fe2O3 magnetic particles formed when the amount of FeCl2 is increased, which is similar to the results reported in the previous COSM.8,9 However, the saturation magnetization decreases when the [FeII]/[FeIII] ratio is greater than 0.046.

3.2. Morphology. The uniform PPy/γ-Fe2O3 composite nanospheres were prepared successfully by using this ICOSM. In order to optimize the formation conditions for the PPy/γFe2O3 composite nanospheres, the molar ratio of p-TSA to Py and the [FeII]/[FeIII] ratio in the reaction solution were investigated. It was found that [TSA]/[Py] molar ratio plays an important role in the formation of PPy/γ-Fe2O3 nanospheres. When [TSA]/[Py] molar ratio is 2, the uniform PPy/γ-Fe2O3 composite nanospheres were successfully prepared by ICOSM 4613

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Figure 6. X-ray diffraction patterns of PPy, PPy/γ-Fe2O3 nanospheres, and γ-Fe2O3: (a) PPy doped with p-TSA; (b) PPy/γ-Fe2O3 nanospheres prepared with [FeII]/[FeIII] ratio of 0.046; (c) PPy/γ-Fe2O3 nanospheres prepared with [FeII]/[FeIII] ratio of 0.023; (d) standard γ-Fe2O3 (JCPDS No. 25-1402).

(Figure 4c). However, no uniform nanospheres can be observed (Figures 4b,d) when the [TSA]/[Py] molar ratio is lower than 1 or higher than 3. It needs to be noted that PPy/γ-Fe2O3 massive aggregates were obtained without any dopant added (Figure 4a). This result shows that p-TSA played a vital role in controlling the nanospheric morphology. Figure 5 shows the SEM images and EDAX spectra of the PPy/γ-Fe2O3 composites synthesized by using ICOSM at different FeCl2 amounts when the [TSA]/[Py] molar ratio was kept at 2. It can be seen that the FeCl2 amounts had no obvious effect on the spherical morphology, and uniform PPy/γ-Fe2O3 composite nanospheres can be obtained for all selected [FeII]/[FeIII] ratios (e.g., 0.014, 0.023, 0.046, and 0.058). Moreover, FeCl2 amounts also had little influence on the diameter of the resulting PPy/γ-Fe2O3 nanospheres. The diameters of all PPy/γ-Fe2O3 nanospheres are ∼80 nm. The EDAX spectra (Figures 5b,d,f, h) of PPy/γ-Fe2O3 composites show that C, N, Cl, S, and Fe, O elements were included in the nanospheres, which identified that the nanospheres are composed of PPy and iron oxide. 3.3. Formation Mechanism of PPy/γ-Fe2O3 Nanospheres. As reported in the Experimental Part, the aqueous FeCl2 solution was added into the white emulsion of pyrrole and ammonia aqueous solution. Pyrrole micelles containing Fe2þ ions might be formed. According to previous reports,14 the above-described micelles are regarded as the “soft templates” to form the electromagnetic composite nanospheres via a self-assembly process. The growth of the PPy nanostructures is allowed by an accretion15 or elongation process.16 It is obvious that the accretion process dominated in this ICOSM. In this process, FeCl3 acts both as the oxidant for the polymerization of pyrrole to PPy and as a source of FeIII for the formation of magnetic γFe2O3. It is essential to add FeCl2 and ammonia at the beginning for the formation of γ-Fe2O3.10 The beginning pH value of the reaction solution is about 11
12 produced by ammonia, which is necessary and beneficial for the formation of Fe3O4 and then γFe2O3. When the FeCl3 was added, Fe3þ reacts with Fe2þ from the preadded FeCl2 to form magnetic γ-Fe2O3 first in the base solution (pH = 11
12). With the addition of FeCl3, pyrrole is also oxidized to PPy and Fe2þ is produced. Although Fe2þ can be

produced with the oxidation polymerization of pyrrole, the Fe2þ produced is not available as a Fe2þ source to form Fe3O4 and then γ-Fe2O3. This is because the pH value of the reaction solution decreased promptly to